The invention relates to portable power tools in general, and more particularly to improvements in power tools of the type wherein a rotary grinding wheel, sander or a like material removing tool is releasably clamped to a motor-driven spindle.
It is known to grip the rotary tool (hereinafter called grinding wheel) between two clamping portions so that the grinding wheel is compelled to rotate with a spindle which is normally driven by a prime mover (such as an electric motor) through the medium of a step-down transmission. One of the clamping portions has internal threads which mate with the external threads of the spindle. It is further known to assemble the spindle from two shafts one of which is hollow and surrounds the other shaft. Reference may be had to German Pat. No. 34 05 885 which further proposes that the two shafts of the composite spindle be rotated as a unit when the two clamping portions bear against and clamp the grinding wheel between them.
Problems arise when the user of the just described power tool attempts to separate the grinding wheel from the spindle. As a rule, the grip of the clamping portions upon the grinding wheel is tightened when the power tool is in use, i.e., the, internally threaded clamping portion is compelled to move toward the other clamping portion and to thus bear against the grinding wheel with a force which cannot be readily overcome by the operator, even if the operator uses a wrench or a like tool.
Attempts to overcome the just discussed problems include the mounting of one of the clamping portions on the hollow (motor-driven) shaft against rotation with reference thereto so that the clamping action upon the grinding wheel increases only if the grinding wheel is caused to turn (i.e., slip) relative to the one clamping portion together with the other clamping portion which mates with the respective shaft. In fact, such mounting of the clamping portions on the composite spindle of a portable power tool is now prescribed by authorities in certain countries including the German Federal Republic. It has been found that, in spite of such safety precautions, after a certain interval of use of the power tool the clamping action upon the grinding wheel is still so pronounced that the removal of the grinding wheel is a cumbersome, strenuous and time-consuming operation.
German Pat. No. 34 05 885 discloses a mode of detaching the grinding wheel from the spindle of a portable power tool practically without the need for any hand tools. The inner shaft of the spindle is stressed by a spring and can be shifted by a thread so as to reduce the clamping force. However, when the patented portable power tool is in actual use, the clamping portions turn relative to each other by friction so that the spring which biases the inner shaft is fully compressed and acts not unlike a rigid body. Once the spring is fully compressed, the grinding wheel can be released only in response to the application of a very large force.
German Utility Model No. 83 17 913 discloses means for arresting the spindle for the purpose of removing the grinding wheel. The user must resort to tools which relax the clamping action upon the grinding wheel while the spindle is held against rotation. The outer clamping disc has holes to allow for penetration of studs on the tool which is used to relax the clamping action.
German Offenlegungsschrift No. 29 48 080 discloses an angular portable grinding tool with means for automatically clamping and releasing the grinding wheel. Such automatic means comprises rod-like motion transmitting elements which extend the full length of the power tool and contribute to the bulk, cost and complexity of the power tool. Moreover, the automatic clamping and releasing means comprises a large number of parts which contributes to initial, maintenance and assembly cost.
Applicants are further aware of the right-angle grinder which is disclosed in German Offenlegungsschrift No. 33 28 955 and is equipped with a safety clutch. The spindle of this power tool also comprises an inner shaft and a hollow outer shaft but the power tool does not embody any specially designed means for facilitating rapid and convenient detachment of the grinding wheel.